Dark current and multipacting trajectories simulations for the RF Photo Gun at PITZ Introduction The PITZ RF Photo Gun Field simulations Dark current simulations Multipacting simulations Summary Igor Isaev CST EUC Strasbourg, 2016
Dark current What Dark Current (DC) is? The DC in an RF accelerating structure is defined as undesirably generated electrons. These electrons are produced and afterwards accelerated with wrong properties in space and time. The DC: produces beam loss that increases radiation level and activation causes acute or chronic damage of accelerator components produces additional unwanted background for users The main source of DC is the field emission of electrons. The field emission current can be parameterized in terms of the modified Fowler-Nordheim equation:, where E is the surface electric field in V/m, Φ is the work function of the emitting material in ev, β is a field enhancement factor, and A e is the effective emitting area. Igor Isaev CST EUC 2016 Slide 2
Multipactor discharge Multipactor discharge (multipacting) is the phenomenon of a resonant secondary electron emission which occurs at certain conditions. Multipactor discharge depends on: Field configuration Cavity geometry Secondary emission yield (SEY) of the cavity material Multipactor discharge corresponds to an exponential growth in number of electrons SEY - Total SEY for copper Incident energy in ev at Incident angle 0 Example of multipactor discharge Exponential growth in number of electrons Example of multipactor trajectories Number of particles Time [sec] Igor Isaev CST EUC 2016 Slide 3
The PITZ RF Photo Gun The RF photo gun operates with a standing wave regime in the π-mode with resonant frequency of 1.3 GHz The gun consists of: normal-conducting cavity (1.6 copper cells) exchangeable molybdenum cathode with CuBe contact spring pair of solenoids Main parameters Accelerating gradient at the cathode, MV/m 60 Beam energy after gun, MeV ~6.8 Full RF power, MW 7 Number of bunches 1..700 RF pulse, µs 800 Repetition rate, Hz 10 Igor Isaev CST EUC 2016 Slide 4
RF and external magnetic fields simulation 1. RF field simulations (CST MWS): Frequency domain solver (F) Eigenmode solver (E) Tetrahedral mesh Half structure symmetry Electric RF field distribution for simulation of the cathode area 2. External magnetostatic fields (CST EM): Magnetostatic solver (Ms-solver) Hexahedral mesh (2 600 000 per ¼) Currents: I main = 350 A, I bucking = -29 A Magnetostatic field distribution Electric RF field distribution for simulation of the gun cavity and the coupler Igor Isaev CST EUC 2016 Slide 5
Dark current simulations Particle sources and settings Example of DC trajectories The PIC solver was selected due to opportunity of the continuous particle emission within a predefined time period. The emission time for the simulations was set to 800 ps, which is longer than RF period (769 ps for the case of 1300 MHz), in order to cover the full range of phases for the dark current emission. Igor Isaev CST EUC 2016 Slide 6
Dark current simulation results Only particles emitted from the cathode and the cathode vicinity can be transported to the beamline and detected by a charge measurement device like Faraday Cup (~30.9% of emitted particles). Energy spectra of dark current Energy spectra is important for calculation of the secondary electrons production probability and estimation of the heat load by DC. Gun part Molybdenum cathode plug Rectangular waveguide Coaxial waveguide Coaxial antenna end Gun cavity Incident particle energies 0.910 ev... 2.73 MeV 0.809 ev... 200 ev 0.318 ev... 3.18 MeV 0.318 ev 6.36 MeV 0.311 ev 6.22 MeV. Igor Isaev CST EUC 2016 Slide 7
Multipacting simulations for the cathode area The number of particles as a function of time The number of particles as a function of time 60 MV/m accelerating gradient at the cathode High probability for the secondary electron emission between the cathode and the blending part of the outer cylinder but There is no continuous growth in the number of particles Igor Isaev CST EUC 2016 Slide 8
Power [dbm] Multipacting simulations for the gun cavity and the gun coupler Multipacting trajectories at accelerating gradient at the cathode of 1 MV/m (~2 kw power in the gun) RF signal High power RF signal (~6 MW) HV RF signal (~2 kw) Multipacting trajectories at accelerating gradient at the cathode of 60 MV/m (~6.5 MW power in the gun) Outer cylinder of the gun cavity time [µs] Surface of the 2-nd gun iris (to coaxial coupler) Coaxial waveguide (between walls of outer conductor) Back wall of the first cell Igor Isaev CST EUC 2016 Slide 9 Multipacting is present in the gun but not preventing operation. 32
Summary > Only particles emitted from the cathode and the cathode vicinity can be detected (~30.9% of emitted particles) > The range of DC particle energies is from 0.3 ev to 6 MeV > There is no monotonic growth in the number of particles at the cathode area. However, the area between the cathode and the blending part of the outer cylinder undergo the secondary electron emission at operating levels of the accelerating gradient of about 60 MV/m > Stable but not multipacting trajectories observed in the gun cavity and the gun coupler parts. Such particles can not induce constant growth of number of electrons but nevertheless could be a reason of surface damage if there exist strong additional particle source Igor Isaev CST EUC 2016 Slide 10
Thank you for your attention. Igor Isaev CST EUC 2016 Slide 11
Spare slides Igor Isaev CST EUC 2016 Slide 12
Example of dark current observations dark current (ua) 250 200 150 100 50 0 p-2-p amplitude Dark current at LOW.FC1 (Pgun=6.5MW, 200us) @ Maximum 5/6/14 31/5/14 26/5/14 21/5/14 16/5/14 11/5/14 6/5/14 1/5/14 26/4/14 21/4/14 16/4/14 11/4/14 6/4/14 1/4/14 27/3/14 22/3/14 17/3/14 12/3/14 7/3/14 2/3/14 25/2/14 20/2/14 15/2/14 10/2/14 5/2/14 31/1/14 26/1/14 21/1/14 16/1/14 11/1/14 6/1/14 1/1/14 27/12/13 22/12/13 17/12/13 12/12/13 7/12/13 2/12/13 27/11/13 22/11/13 17/11/13 12/11/13 7/11/13 2/11/13 28/10/13 23/10/13 18/10/13 13/10/13 Igor Isaev CST EUC 2016 Slide 13
Gun DC@450A Gun DC@430A Gun DC@400A Gun DC@350A Igor Isaev CST EUC 2016 Slide 14
Dark current measurements for different guns Maximum dark current [µa] 600 500 400 300 200 Gun4.2, 2008-08-31, Mo#113.1 Gun4.2, 2008-09-15, Cs2Te#23.3 Gun4.1, 2011-06-06N, Cs2Te #625.1 Gun4.1, 2012-01-03A, Mo #133.1 Gun 3.1, 2013-02-21 Cs2Te #149.1 Gun 4.3, 2013-06-08, Cs2Te, early cond. stage Gun 4.3, 2013-07-14 Cs2Te, late cond. stage Gun 4.4, 2013-11-24, Mo Gun 4.4,2013-11-28, Cs2Te #126.2 100 0 3 3,5 4 4,5 5 5,5 6 6,5 7 7,5 8 RF power in the gun [MW] Igor Isaev CST EUC 2016 Slide 15
Multipacting measurements RF reflected power signals Dark current Faraday cup signal Multipacting Solenoid scan maps from multipacting measurements Igor Isaev CST EUC 2016 Slide 16